1. Field of the Invention
The present invention relates to a cross coil type analog indicating instrument and more particularly to a driving apparatus suited for driving the analog indicating instrument concerned.
2. Description of the Related Arts
In a driving apparatus of this type has been heretofore normally furnished with a frequency-voltage converter for converting a car speed into analog voltage. The frequency-voltage converter receives a signal having a frequency proportional to a car speed generated by a car speed pickup and converts the signal into an analog voltage proportional to the frequency. And the analog voltage supplies cross coils with an electric current so as to drive them, thereby to indicate the car speed by a pointer coupled with the cross coils.
In such a structure, ripple components are usually contained in the analog voltage outputted from above-mentioned frequency-voltage converter. Since these ripple components are conspicuous particularly in a low frequency region, such a problem is posed that large pointer deflection phenomenon presents itself on the pointer.
In regard to the above, it is conceivable to adopt a smoothing capacitor having large electrostatic capacity for increasing smoothing degree of the ripple components, thereby to restrain above-mentioned large pointer deflection phenomenon. In such a case, however, instead of a fact that large pointer deflection phenomenon in a low frequency region can be restrained, above-mentioned smoothing causes delay in response in the output of the frequency-voltage converter, and the pointer cannot deflect with good responsibility following kaleidoscopic change of the car speed. As a result, such a problem is posed that correct car speed information cannot be brought to a crew.
While a smaller electric capacity of the capacitor may improve the responsibility by reducing the smoothing degree of ripple components, the capacitor cannot dissolve the pointer deflection phenomenon in the low frequency region. As a result, such a problem is posed that visual confirmation of speed information is not easy.
Kobayashi et al. disclose a driving apparatus by a digital circuit which generates an instrument driving signal having a suitable time constant by using a frequency-voltage (F-V) converter section selected in accordance with a frequency of an input pulse in JP-A-1-118772. It is also conceivable that above-mentioned frequency-voltage converter is structured with a digital circuit as disclosed by Kobayashi et al., but the frequency-voltage converter becomes larger in size and higher in cost.
Further, according to an apparatus for driving a pointer in accordance with a trapezoidal waveform stored in advance corresponding to an input signal level, the deflection of a pointer caused by ripples is eliminated, but the deflection caused by a point of inflection of the trapezoidal waveform is generated. The present inventors have disclosed in JP-A-2-222840 driving means which generates a triangular wave in accordance with an analog input, and supplies the triangular wave to the cross coils after having the triangular wave approximate to a sine wave. With this, the deflection of the pointer becomes smooth, but responsibility in a low frequency region is insufficient.